A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

learn more… | top users | synonyms (1)

-1
votes
1answer
57 views

Gravitational Lensing Diffrence between Big and Small Black Holes

Would you see more of the back holes iris in smaller black holes compared to larger ones and if so can one be big enough the photon sphere would completely cover it and it would look like distorted ...
0
votes
0answers
24 views

Clarification on hawking radiation [duplicate]

I read that a pair of entangled quantum particles can spontaneously pop into existence for a brief moment and especially near to the event horizon whereby one of the entangled quantum particle passes ...
6
votes
2answers
1k views

Is time going backwards beyond the event horizon of a black hole?

For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside. Is this the case? This question is a followup ...
3
votes
0answers
134 views

Are gravastars observationally distinguishable from black holes?

Are observations of Hawking radiation at the acoustic event horizon in Bose-Einstein condensates consistent with gravastars? To reconcile the second law of thermodynamics with the existence of a ...
1
vote
1answer
51 views

What does singularity mean in the context of black holes? [duplicate]

Non-rotating big stars can be subject to a gravitational collapse increasing their density. When the density is so high that the mass volume shrinks below the event horizon, a black hole is formed. ...
2
votes
1answer
48 views

Can many universes exist inside black holes? [closed]

As temperature and pressure grows, matter changes. The energy imposed on matter forces particles to disintegrate and recombine in different ways and subatomic orbitals fold over each other into new, ...
0
votes
1answer
36 views

Is the information that a human brain (and body) “contains” superflorous to the information contained within the atoms of the body?

Layman here. I recently watched this video talking about Stephen Hawking's theory of hairy black holes. The speaker mentions, as I've heard before, that once "something" has entered a blackhole its ...
3
votes
0answers
40 views

Special relativity and black hole paradox [duplicate]

Today a friend of mine posed me a paradox involving black-holes, one that I couldn't solve. Suppose we have planet, with a density such that it is almost to the point of turning into a black hole, ...
1
vote
1answer
94 views

Can you shine a laser on an object that has fallen into a black hole? [duplicate]

I've been having a small back and forth on another website about the nature of objects that fall into black holes. I know that they never reach the event horizon from the perspective of a distant ...
2
votes
2answers
94 views

Gravitational lensing and perspective

When looking at a black hole the ability to see ourselves is compromised by the small window for the light trajectory to come back to us. The light from the Earth that is bent by (almost) 180° and ...
1
vote
1answer
78 views

Time independent Kerr metric

The Kerr metric expressed in terms of polar coordinates $r,\theta,\phi$, such that $x = r\sin(\theta)\cos(\phi)$, $y = r\sin(\theta)\sin(\phi)$, $z = r\cos(\theta)$. Then the Kerr metric is given as ...
1
vote
3answers
170 views

Path of light as it travels between two black holes

What would happen to light passing through a narrow space between the event horizons of two equal-mass black holes? Would it deviate or follow a straight path?
1
vote
0answers
33 views

What are the difference between a black holes and a black branes? [closed]

I want to know the fundamental difference between a black hole and a black brane? What is the planar limit?
1
vote
1answer
309 views

Does matter shrink uniformly without restriction in freefall to central region of black hole?

It is often stated that a distant observer will observe a freefalling object as experiencing an infinite journey to the event horizon, but that a traveler in the local frame will experience a ...
17
votes
7answers
1k views

Do all massive bodies emit Hawking radiation?

It is known that any accelerated observer is subject to a heat bath due to Unruh radiation. The principle of equivalence suggests that any stationary observer on the surface of a massive body should ...
1
vote
5answers
229 views

Can we speed up the evaporation of black holes manually by accelerating it?

If we throw an object to pass near a black hole, to bypass it, it will change the speed of the black hole, just like gravitational assist for a space probe. Does an accelerating black hole evaporate ...
6
votes
1answer
174 views

Can we tell a wormhole from a black hole by observation?

I'm assuming stable wormhole really exist (naturally/microscopic or not) how can we distinguish between a wormhole and a black hole since we can't probe their "insides"?
3
votes
0answers
47 views

What is the inertial mass of a black hole?

Or the inertial mass of any spherically symmetric object, can I calculated by measuring very accurately the spacetime distortion this object produces in its surroundings? With 'inertial mass' I mean ...
20
votes
5answers
6k views

How to get Planck length

I know that what Planck length equals to. The first question is, how do you get the formula $$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length? The second question is, will any ...
0
votes
1answer
52 views

Acceleration of a static observer in Schwarzschild measured in locally inertial frame [closed]

Consider a static observer at $r=R>2M$ with four velocity given by $$U^\mu=\frac{dx^\mu}{d\tau}=(1-\frac{2M}{R})^{-1/2}\delta^\mu_0$$ in the Schwarzschild metric ...
5
votes
1answer
109 views

Simple metric of stellar collapse

Is there a simple metric (Lorentzian manifold) known which exhibits the formation of a black hole while not having any white hole counterpart and which moreover satisfies the strong and dominant ...
0
votes
3answers
214 views

Do black holes violate the uncertainty principle?

If black holes have mass but no size, does that imply zero uncertainty in position? If so, what does that imply for uncertainty in momentum?
0
votes
0answers
21 views

How do black holes form without violating the uncertainty principle? [duplicate]

Many videos on YouTube while discussing black holes mention that it's born out of a heavy star when it collapses into a single point and that infinitely curves spacetime around it. When all the mass ...
1
vote
1answer
68 views

How destructive would a small black hole be on the surface of the Earth?

Say a black hole with the diameter of around 200 meters or so was formed on the Earth's surface, how much damage would it cause?
11
votes
3answers
982 views

Why does a singularity need to exist at the center of a black hole?

Sorry if this is a very basic question, but I am not a physicist, but this is something that has always intrigued me. I have read that the properties of a black hole are its mass, spin and angular ...
3
votes
2answers
56 views

Is all of a black hole's mass situated arbitrarily close to the event horizon?

Forgive me if I'm thinking about black holes in completely the wrong way, but since time dilation increases to arbitrarily large amounts the closer you get to the event horizon of a black hole, ...
3
votes
1answer
221 views

Beyond the gravity of a black hole

http://www.sciencealert.com/the-magnetic-field-just-outside-our-black-hole-has-been-studied-for-the-first-time reported strong magnetic fields escape out of black holes. Does that rule out photons ...
0
votes
2answers
352 views

Does the sun have a black hole inside of it?

The Schwarzschild radius of the sun is approx. 3 km. It would be a black hole if all the mass of it were within the 3km. However, even with the mass spread out couldn't the sun technically have a or ...
4
votes
3answers
386 views

Zero $g$ in a black hole?

At the very centre of the Earth, masses cancel each other out, creating an effective zero-$g$ environment. Would the same happen in a black hole?
3
votes
1answer
81 views

The Riemannian Penrose Inequality in higher dimensions

I am reading the proof of the Riemannian Penrose Inequality by Huisken and Ilmamen in The Inverse Mean Curvature Flow and the Riemannian Penrose Inequality and I was wondering why they restrict their ...
2
votes
2answers
186 views

The black hole paradox

I recently read in the news that Stephen Hawking claims to have solved the Black Hole Information paradox. I researched a bit about the paradox and the research that Stephen Hawking did to solve it. ...
2
votes
1answer
82 views

Generalized Bekenstein-Hawking temperature for Kerr-Newmann-dS black holes

What is the formula for the Bekenstein-Hawking temperature in a Kerr-Newmann-de Sitter spacetime, i.e., the temperature for a black hole with Mass (M), angular momentum (J), electric charge (Q) and ...
0
votes
1answer
50 views

See behind the black hole

Why in this video does the 2nd black hole appears to change size and appear larger the farther away it gets? How can you see behind it? http://www.youtube.com/watch?v=ENd8Sz0AFOk
0
votes
0answers
63 views

Black hole and relativity [duplicate]

This link shows movement around the black whole. It seems impossible for them to move this fast. http://www.youtube.com/watch?v=ENd8Sz0AFOk If we were able to watch a ship of people go into a black ...
4
votes
3answers
466 views

Can a neutron star compress until it's converted to a black hole?

The Universe's densest objects are black holes. In the second place, there are neutron stars. So, if a neutron star compresses to its Schwarzschild radius, would it appear as a black hole? That black ...
7
votes
1answer
80 views

Lower limits for steady-state black holes

Stellar mass (and larger) black holes emit Hawking radiation below the temperature of the cosmic microwave background; thus, they should always absorb more energy from space than they emit, and always ...
1
vote
1answer
47 views

Do zero gravitational-force points produce Naked singularities?

In between two singularities, does a particle experience gravity? If a particle is exactly at the midpoint between two singularities, will its net experience of the gravitational force (ignoring the ...
1
vote
0answers
14 views

Effect on mass near a black hole [duplicate]

When an object falls inside the event horizon with already huge terminal velocity directed towards the black hole, the gravity must accelerate to near light speeds which should almost increase its ...
1
vote
0answers
60 views

Not even light can escape black hole due to it's pathetic speed or infinite time dilation or extreme curvature of space? [closed]

The correct explanation that why black hole is black must be one of the following: (1) Escape velocity of black hole exceeds speed of light like a pitched baseball can never go into orbit. (2) From ...
1
vote
4answers
54 views

Light from a person falling into a black hole getting dimmer

I have read that to an observer at rest outside a black hole, they will see the light from the free faller get redshifted and dimmer. What is the mathematical explanation for the light actually ...
9
votes
2answers
317 views

Information Preservation and Burning Books

I recently read an article in the NY Times called A Black Hole Mystery Wrapped in a Firewall Paradox. I really liked the article, but reading one quote immediately made me think of asking Physics.SE a ...
1
vote
0answers
39 views

Gauss-Bonnet correction to Carnot's efficiency

Could any one tell explicitly, how did the author get $Tc$ Vs $\alpha$ Plot given in Figure 3 in Gauss-Bonnet Black Holes and Holographic Heat Engines Beyond Large N
0
votes
0answers
16 views

Is black hole a black body? [duplicate]

I have studied in my physics class today that black body is one which absorbs radiation of all wavelength incident on it , Its absorptance is unity , reflectance and transmittance is zero . I wonder ...
1
vote
3answers
1k views

How do we determine the temperature of a Black Hole?

How do we determine the temperature of a Black Hole? Since we cannot see a Black Hole, which I presume, is because it absorbs light, would it not also prevent radiation from escaping, making ...
5
votes
2answers
204 views

Can a black hole have negative temperature?

Stephen Hawking said that black hole also have temperature and it is related to its mass so in other words a black hole can also be shown to have a negative temperature! I know that nothing is colder ...
4
votes
0answers
268 views

Are Black Holes set to take over the Harmonic Oscillator in the 21st century? [closed]

A few years ago I attended a talk given by Andy Strominger entitled Black Holes- The Harmonic Oscillators of the 21st Century. This talk, ...
0
votes
0answers
34 views

What would you experience if the universe ceased existing whilst you were under extreme time dilation?

Hypothetical situation, if you were in the center of a supermassive black hole (and managed to not be immediately obliterated), time dilation would be so that billions of years may pass outside the ...
-4
votes
1answer
96 views

do we live in 3.5 dimension? [closed]

After watching interstellar movie i came to know that time is 4th dimension and watch some videos and read articles which says there is there is around 10 dimension(11 also). As x,y,z are 3 dimension ...
9
votes
3answers
222 views

Coupling between galaxy spin and central black hole spin

What is the relationship between the spin of a galaxy and the spin of its corresponding black hole? Associated questions: Do they always have the same axis of rotation? Do they always spin in the ...
2
votes
1answer
1k views

A two-dimensional event horizon?

Today, I came across an article which says that the Universe began with the collapse of a 4-dimensional [spatial] Universe. Well, I don't know if there's any way to confirm these like it is with all ...